The Formation Age of Carbonates in ALH 84001. M.Wadhwa1 and G. W. Lugmair2, 1Department of Geology, The Field Museum, Roosevelt Road at Lake Shore Drive, Chicago IL 60605, USA, 2Scripps Institute of Oceanography, University of California at San Diego, La Jolla CA 92093, USA.

Published in Meteoritics, 31, p. A157.

Among the SNC (martian) meteorites, ALH 84001 is particularly interesting because it has a significantly older crystallization age [1,2]. Moreover, it contains abundant carbonates believed to have formed from interaction with CO2-rich aqueous fluids in a near-surface environment on Mars [3,4]. The C isotopic composition of these carbonates suggests that their parent fluids carried the signature of a highly fractionated martian atmosphere [5]. The formation time of these carbonates would then be important in constraining how early (or late) in the geologic history of Mars this atmospheric fractionation occurred. Therefore, to obtain a formation age for carbonates in this meteorite, we have measured Rb and Sr concentrations and Sr isotopic ratios in bulk samples and mineral separates of ALH 84001.

On a plot of 87Sr/86Sr vs. 87Rb/86Sr the bulk samples, as well as the pyroxene mineral separates, fall on a best-fit line corresponding to an age of 3.84 ± 0.05 G.y., which is considerably lower than what an earlier report implies [2]. The age recorded by the Rb-Sr system is similar to the Ar-Ar shock age [6], and therefore most likely reflects the time of intense shock experienced by ALH 84001. The maskelynite and carbonate data points, however, are slightly offset from the 3.84-G.y. isochron defined by the bulk samples and the pyroxene separates. This is, in fact, to be expected if the carbonate was formed by partial replacement of the original plagioclase (as evidenced by petrographic features in this meteorite[3,4]), and this event occurred significantly later than the last equilibration of the Rb-Sr system in ALH 84001. The age of formation of the carbonates can be determined from the slope of the line joining the carbonate and the maskelynite data points and was calculated to be 1.39 ± 0.10 G.y.

There are several implications of a 1.39 ± 0.10-G.y. age for the carbonates in the ALH 84001 meteorite. The most obvious one is that it provides yet another piece of evidence in support of a martian origin for ALH 84001, since nowhere else in the inner solar system (except Earth) is it reasonable to assume the presence of aqueous fluids at that time. Moreover, all other SNC meteorites known to contain secondary aqueous alteration minerals have formation ages <=1.3 b.y., and thus 1.39 G.y. is so far the oldest time in the geologic history of Mars for which the presence of CO2-rich aqueous fluids actively percolating though crustal rocks is now firmly established.

A 1.39-G.y. age for carbonates also has implications for the isotopic evolution of the martian atmosphere. Given the C isotopic composition of these carbonates [5], our results imply that there was only minimal isotopic fractionation of the atmosphere on Mars, at least within the last 1.39 G.y. Finally, the similarity of this carbonate formation age to the 1.3-G.y. crystallization age for some of the other SNC meteorites could be indicative of a particularly active phase of magmatism and hydrothermal activity on Mars at that time. Although this can only be regarded as speculative at this point, it does raise some interesting possibilities regarding the magmatic history of this planet.